Apparatus for administration of at least two gases to a patient

ABSTRACT

An apparatus for administration of at least two gases to a patient is described. The apparatus comprises a patient circuit (40) which consists of a circle system for rebreathing and a drive circuit (20), and which is provided with delivery means (45, 46) for supplying fresh gas, and a carbon dioxide absorber (44), the apparatus further comprising an actual value transducer (47) for the gas concentration of one gas type, a desired value transducer (81) for indicating the desired gas concentration, and a control device (80) for maintaining said gas concentration. The apparatus is characterised in that the patient circuit and the drive circuit communicate with each other via a gas switching unit or exchanger (30) for open separation, and that the control device (80) comprise a calculating means for determining the gas volume and gas type to be supplied to the patient circuit (40) so as to obtain the desired value concentration.

The invention relates to an apparatus for administration of at least twogases to a patient, according to the preamble of claim 1.

Such an apparatus for anaesthetic purposes is disclosed in U.S. Pat.specification No. 4,127,121. The circle system with the absorberprovides for rebreathing and thus brings the advantage that expensivegases are saved and the loss of moisture and heat is minimised.Moreover, environmental hygiene aspects necessitate keeping down theconsumption of anaesthetic gases. The expiration gas is supplied to abellows which forms part of the drive circuit and presses the gases backto the patient's lungs via the carbon dioxide absorber. The patientcircuit and the drive circuit form a closed system.

According to the above-mentioned patent specification, both theconcentration of one gas type and the volume variation in the systemmust be measured so that a desired gas mixture with a minimumconsumption of anaesthetic gases can be maintained. It appears from thispatent specification that the fresh gas is supplied continuously, whichimplies that considerable amounts of excess gas must be exhausted fromthe patient circuit via a valve (not shown).

Rebreathing is essential, when expensive gases are administered, such asanaesthetic gases, helium which is used for treating severe cases ofasthma, xenon which is used as contrast medium in computer tomographyand as anaesthetic gas, etc. At the same time, respirator ventilationunder anaesthetic can be highly advantageous in that, on the one hand,the anaesthetist and/or the nurse is free to perform other importantduties and, on the other hand, the respirator is a better means thanmanual ventilation for establishing an optimal patient breathingpattern. In the treatment of asthma, the respirator can be of vitalimportance.

One prior art closed system is shown in FIG. 1a. FIG. 1a shows a closedsystem with an anesthetic bag 1 in communication with an excess valve 2.The excess valve 2, in turn, is in communication with a CO₂ absorberwhich is in communication with a vaporizer 4. The vaporizer 4 suppliesfresh gas to a patient 5. Elements common to FIGS. 1a, 1b and 1c havethe same reference numeral.

One possibility of combining the respirator circuit and the anaestheticcircuit is illustrated in FIG. 1b wherein a respirator 6 is made todrive a bellows 7 which is enclosed in a container 8 and operates theanaesthetic circuit. This is the so-called bag-in-bottle principle whichcan also be built into the respirator 6 proper. However, thebag-in-bottle principle comprises some special arrangements; on the onehand there is the arrangement which has given the method its name and,on the other hand, the function of the excess valve 2 must be checked.

A further possibility of combining the respirator circuit (drivecircuit) and the patient circuit (receiver a circuit) is to connectbetween them a switching unit 9 or exchanger which openly separates thegases in the two circuits as illustrated in FIG. 1e. During inhalation,the gas of the respirator 6 (e.g. oxygen) is supplied to the exchanger.The pressure is propagated to the patient circuit for operation thereof.During expiration, the gas flows back. In its simplest form, theexchanger may be an empty container. The best way is, however, to designthe exchanger such that the front therein, between respirator gas andpatient gas, becomes as well defined as possible. This can be achievedby giving the exchange the form of a tube.

An apparatus for inhalation anaesthesia comprising a gas exchangerbetween the drive circuit and the patient circuit is described in thepaper "A new generation of anaesthetic ventilators" by A. P. Adams andJ. D. Henville in Anaesthesia 1977, Vol. 32, pp. 34-40. The describedexchanger is in the form of a corrugated tube, and serves to avoiddilution of anaesthetic gas. The fresh gases for anaesthetic andinhalation are supplied continuously and controlled by a flow meter.

The object of the invention is to control in a simple manner the gasconcentration in the patient circuit with a minimum supply of fresh gas,thereby to improve the saving of e.g. expensive anaesthetic gas. Theobject is achieved by means of a communication, stated in thecharacterising clause of claim 1, between the patient circuit and thedrive circuit via a or exchanger, and a control device calculating andsupplying fresh gas volumes to the circle in response to the desiredvalue and actual value concentrations of a gas in the patient circuit.

The invention is based on the surprising discovery that a system withopen separation between the patient and drive circuits obtains aninherent equilibrium so that the gas concentration in the patientcircuit is more readily controllable by the supply of small amounts ofone or the other gas type, depending on whether the gas concentration istoo low or too high. Only this insight renders it possible to minimisethe consumption of expensive anaesthetic gas, such as xenon, withoutmeasuring the volume variations in the patient circuit.

This important simplification makes it possible to effectively control,by electronic means, the gas concentration in the patient circuit andfacilitates the anaesthetist's work, particularly during longanaesthesia, while simultaneously providing a highly reliable systemensuring the patient's safety.

The invention thus allows a very small supply of fresh gas, whilemaintaining full safety for the patient. If, for some reason, the supplyof fresh gas to the patient circuit should be too low, a given amount ofoxygen is administered automatically to the patient via the exchangerfrom the drive circuit. Further, because of the small supply of freshgas, the invention also causes the amount of anaesthetic gas escapingfrom the patient circuit restricted to the small amount which passes viathe exchanger into the drive circuit and thus is effectively recovered.

Further advantages obtained by using the exchanger are, inter alia, thatthe connection between the respirator circuit and the patient circuitbecomes simple and at the same time inexpensive, and that the excess gasfrom the patient circuit is recovered via the respirator. Moreover,exchanger functions as a buffer for volume variation in the patient'slungs.

It is also easier than before to use a so-called closed system. If theflow of fresh gas is shut off, a net flow towards the patient occursthrough the exchanger owing to the patient's oxygen consumption. Thefresh gas is then automatically replaced by oxygen which flows from therespirator part. In a conventional closed system, the fresh gas flowmust continuously be balanced in relation to the oxygen content andvolume in the patient circuit.

The invention will now be described in detail below, reference being hadto the drawing in which:

FIG. 1A illustrates a prior art closed circle system with an anaestheticgas bag;

FIG. 1B shows the same system in a "bag-in-bottle" variant and arespirator as the drive circuit;

FIG. 1C shows the circle system with a switching unit or exchanger foropen separation, and the respirator as the drive circuit;

FIG. 2 illustrates a first embodiment of the inventive apparatus forinhalation anaesthesia;

FIG. 3 is the same as FIG. 2, except for the drive circuit conduit forrecovery of anaesthetic gas;

FIG. 4 is also the same as FIG. 2, except for the drive circuit conduitfor recovery of anaesthetic gas; and

FIGS. 5-7 illustrate a flow chart for data control of the fresh gassupply.

According to the embodiment shown in FIG. 2, the apparatus is dividedinto two parts, i.e. the drive circuit 20 and the patient circuit 40.The two circuits communicate with each other via the exchanger 30 whichis a tube having an internal volume of 2-3 liters. The exchangerprevents mixing of the gases in the two circuits. The drive circuit 20contains at least 85% O₂, and the remainder is anaesthetic gas, such asxenon, which is gradually accumulated. The drive circuit 20 consists ofe.g. a Siemens-Elema Ventilator 900. The drive circuit further comprisesa conduit 21 for supplying oxygen when necessary, and an oxygen meter22. The drive circuit is also provided with a return flow conduit 50which comprises a two-way valve 51 between the atmosphere and the drivecircuit. The two-way valve 51 is open to the atmosphere until theanaesthetic gas supply is started. The two-way valve 51 is followed by awater absorber 52 for eliminating technical difficulties caused bymoisture. An additional absorber 53 may be arranged, if required, toprevent chemical attack on the equipment. The absorber 53 absorbs gaseswhich are aggressive towards metal and plastics in the apparatus, forexample halothane and/or isoflurane. A compressor 54 is arranged toincrease the pressure from 0 to about 1-7 atm. gauge, and is followedfirst by an intermediate nonreturn valve 55, and then by a high-pressurebuffer 56. The high-pressure buffer 56 absorbs volume increases in theentire system. Such increases can occur, when a sudden change of the gascontent in the patient circuit is required. However, the buffer usuallyoperates at its lower limit of 1 atm. gauge, since more oxygen isconsumed than supplied via a valve 46. The high-pressure buffer 56 alsoincludes a safety valve 57 which opens at 7 atm. gauge. Thehigh-pressure buffer 56 is followed by an oxygen meter 58 which in turnis followed by an outlet valve 59 adjacent the high-pressure buffer. Theoutlet valve 59 maintains the buffer pressure at the desired value. Atwo-way valve 60 between the gas bottle and drive circuit opens to thebottle, when the oxygen content in the drive circuit is lower than agiven value (e.g. 85%) and continues to be open until the value hasrisen to another given value (e.g. 95%). The gas bottle 62 facilitatesrecovery of the anaesthetic gas. Between the two-way valve 60 and thegas bottle 62, there is a high-pressure compressor 61 for suppling thebottle 62.

The patient circuit 40 comprises an inhalation leg 41, an expiration leg42 and a patient insert member 43. A carbon dioxide absorber 44 isarranged in the inhalation leg 41 adjacent the exchanger 30. In theproximity of the carbon dioxide absorber 44, there are arranged solenoidvalves 45 and 46 for supplying anaesthetic gas and oxygen, respectively.In the proximity of the solenoid valves 45 and 46, there are arranged anoxygen meter 47 which is an actual value transducer for the oxygenconcentration, and an oxygen meter 48 for checking purposes. Thesemeters may be included in the patient insert member 43, but may also bearranged at a different point in the patient circuit. Also the solenoidvalves 45 and 46 can be alternately positioned and can be connected bothin the proximity of and at a distance from the carbon dioxide absorber44. The inhalation leg 41 and the expiration leg 42 are provided withsuitably positioned nonreturn valves 49. It thus appears from the abovedescription that the drive circuit serves four functions, viz.:

(a) it functions as a ventilator and thus operates the lungs,

(b) it is a source of oxygen,

(c) it buffers volume variations in the entire system, and

(d) it accumulates xenon and when the content has reached a given value,the gas mixture is supplied to an accumulator bottle for recovery lateron.

Below follows a description of the function of the apparatus which iscontrolled by a microprocessor 80 receiving input signals from the O₂meter 47 and the desired value transducer 81 via lines 470 and 810,respectively. The output signals from the microprocessor are deliveredvia lines 801 and 802 of the solenoid valves 45 and 46. First, thecircle system comprising the exchanger 30 and the patient circuit 40 isfilled with oxygen by a supply of for instance 2-10 liters O₂ per minutevia the solenoid valve 46. The two-way valve 51 has been set in theatmosphere position. The desired oxygen concentration is set, and at thesame time a number of check-ups of the apparatus 10 are carried out bythe microprocessor according to the flow chart shown in FIG. 5.

At block 210, of the flowchart level 1, the first function carried outby the microprocessor is to initiate I/O and variables of the system.

At block 220, the operator is interrogated about tidal volume, FRCvolume, circle volume, maximum pressure in high-pressure buffer andminimum oxygen content in high-pressure buffer.

At block 230, the microprocessor checks whether a key has been pressed;

At block 240, the microprocessor checks pressure and O₂ in thehigh-pressure buffer.

At block 250, the microprocessor checks the patient's O₂ concentration.

Finally, at block 260, the microprocessor checks whether there arefurther boluses to supply.

Based on the known volume of the circle system and the patient's assumedlung volume as well as the assumed virtual distribution volume of theanaesthetic gas in the patient's body, which together constitute themixing volume, the microprocessor calculates the amount of anaestheticgas required to obtain the intended oxygen content. To avoid hypoxia,the calculation is made by a certain margin. The calculated anaestheticgas volume is infused, and the excess gas is exhausted via the exchanger30.

After starting up, the gas supply is controlled via the solenoid valves45 and 46, based on the O₂ concentration as read from the meter 47. Asshown in FIG. 6, there is also a time control unit in box 100 forcontrolling the interval in the fresh gas supply. The interval is chosenby the operator when the microprocessor is initiated.

As appears from FIG. 6, the actual value of the oxygen content in thecircle system is now determined by the oxygen meter 47. Subsequently,the difference between the desired oxygen concentration and the actualoxygen concentration is calculated in box 120. If this difference isless than the hysteresis of the O₂ concentration, xenon is suppliedaccording to boxes 130 to 150 in the following manner: if xenon wassupplied at the previous measurement, the regulator amplificationincreases by 50%. If oxygen was supplied at the previous measurement,the regulator amplification decreases by 50%. The volume to be suppliedis calculated according to the equation: absolute value ln (desired O₂concentration/present O₂ concentration)×(tidal volume+residualvolume+volume of the circle system)×(regulator amplification), where thevolume supplied is allowed to be ten times the tidal volume at most. Thevolume is supplied via the solenoid valve, if the volume is not toosmall (about 10 ml). If, however, the above difference is greater thanthe hysteresis of the O₂ concentration, O₂ is supplied as follows: ifoxygen was supplied at the previous measurement, the regulatoramplification increases by 50%. If xenon was supplied at the previousmeasurement, the regulator amplification decreases by 50%. The volume tobe supplied is calculated according to the equation above. The oxygenvolume is supplied, if the volume is not too small (about 10 ml). Theoxygen volume is supplied to the circle system as described in boxes 160to 200. The description of the adjustment of the regulator amplificationis illustrated in FIG. 7, boxes 310 to 340.

In the return flow conduit 50 of the drive circuit, the anaesthetic gasis accumulated, after water has been separated in the absorber 52, andpollution gases in the form of e.g. halothane have been separated in theabsorber 53. The pressure of the returning gas is increased in thecompressor 54, and this gas is supplied to the accumulator bottle 62 viathe high-pressure compressor 61, when the anaesthetic gas contentexceeds a given value. Should the pressure at the outlet valve 59 of thehigh-pressure buffer 56 drop so far that the ventilator function of thedrive circuit cannot be maintained, oxygen is supplied via the conduit21. When the accumulator bottle 62 is full, it can be used forrecovering anaesthetic gas in a recovery unit.

FIG. 3 illustrates an alternative embodiment for recovering anaestheticgas in the return flow conduit 50 of the drive circuit. In thisalternative, only one compressor is required, and the compressor 502supplies both the bottle 62 and the high-pressure buffer 56. Thiscompressor must have a suction effect to propel the gas through the twoabsorbers 52 and 53. The low-pressure buffer 501 comprises a bellows(about 1-2 liters).

A further alternative for recovering anaesthetic gas is shown in FIG. 4.When the content of the anaesthetic gas (e.g. xenon) is very low, thegas can be processed in a zeolite filter, whereby the temperature isreduced and the pressure rises. The outlet gas from the drive circuit 20is discharged through the outlet conduit 70 where it is purified ofcarbon dioxide, water vapour and, possibly, high-molecular gases(isofluran type) in absorbers 71, 72, 73 for the respective substance.The remaining gas, i.e. O₂ with an admixture of xenon, is compressed inthe compressor 74, cooled in the cooling unit 75 and enters a zeolitefilter 76, where the xenon is trapped. Additional oxygen can optionallybe supplied at the end of the process to ensure that the gas mixture inthe zeolite container consists of xenon-oxygen. Then the filter can beprocessed for recovering xenon.

The exchanger may be alternatively designed, in its simplest form as atube. The exchanger may also comprise a plurality of tubes connected inparallel or in series. The exchanger can also be filled with corrugated,folded and/or laminated material or like material, and also be filledwith balls or pellets, and can further, or alternatively, be filled withporous material, fibers, metal shavings mesh or the like.

The embodiments shown illustrate the value of the invention forinhalation anaesthesia. In the same advantageous manner, the inventionmay be used for, inter alia, computer tomography, where xenon is used asa contrast medium.

I claim:
 1. An apparatus for administration of at least two gases to apatient, said apparatus comprising a patient circuit (40) which consistsof a circle system for rebreathing and a drive circuit (20), and whichis provided with delivery means (45, 46) for supplying fresh gas, and acarbon dioxide absorber (44), said apparatus further comprising anactual value transducer (47) for the gas concentration of one gas type,a desired value transducer (81) for indicating the desired gasconcentration, and a control device (80) for maintaining said gasconcentration, characterised in that said patient circuit and said drivecircuit communicate with each other via an exchanger (30) for openseparation, and that said control device (80) comprises a calculatingmeans for determining the gas volume and gas type to be supplied to saidpatient circuit (40) so as to obtain the desired value concentration. 2.The apparatus as claimed in claim 1, characterised in that the gasvolume is determined in such manner that the desired value concentrationis obtained by a minimum supply of fresh gas.
 3. The apparatus asclaimed in claim 1 or 2, characterised in that the gas volume issupplied by pulses to said patient circuit (40).
 4. The apparatus asclaimed in claim 3, characterised in that the gas pulses are supplied tosaid patient circuit (40) at large intervals, for example one minute. 5.The apparatus as claimed in any one of claims 1 or 2, characterised inthat fresh gas is supplied at different times for different gas types.6. The apparatus as claimed in any one of claims 1 or 2, characterisedin that a computer is included in said control device for calculatingand/or checking the supply of gas and/or concentration of gas in saidpatient circuit.
 7. The apparatus as claimed in claims 1 or 2,characterised in that one gas type is xenon.
 8. The apparatus as claimedin claims 1 or 2, characterised in that said exchanger (30) comprises atubular container.
 9. The apparatus as claimed in claim 5, characterisedin that said container comprises a plurality of tubes connected inseries or in parallel.
 10. The apparatus as claimed in claim 8,characterised in that said container is filled with corrugated, folded,and/or laminated material.
 11. The apparatus as claimed in claim 8characterised in that said container is internally provided with a mesh.12. The apparatus as claimed in claim 1, characterised in that saidcalculating means determines the gas volume and gas type based on thedifference between the desired value and actual value concentration ofthe gas type.
 13. The apparatus as claimed in claim 1, characterised inthat said calculating means determines the gas volume to be supplied inproportion to a logarithmic function for the quota between the desiredvalue and actual value concentration of the gas type.
 14. The apparatusas claimed in claim 12 or 13, characterised in that said calculatingmeans determines the volume to be supplied as the absolute value ln(desired gas concentration/present gas concentration)×(mixingvolume)×(regulator amplification).
 15. The apparatus as claimed in claim14, characterised in that said calculating means sets up a test valuefor the mixing volume and adjusts said value by changing the factor forthe regulator amplification in response to the measured gasconcentration after the calculated volume has been added.
 16. Theapparatus as claimed in any one of claims 12 or 13, characterised inthat sasid calculating means comprises a microprocessor (80).
 17. Theapparatus as claimed in claims 1 or 2 or 12 or 13, characterised in thatexpiration gas passing the drive circuit side of said exchanger (30)during expiration, is delivered back to the drive circuit via a returnflow conduit (50).
 18. The apparatus as claimed in claims 1 or 2 or 12or 13, characterised in that said conduit comprises a compressor (54)for compressing the expiration gas.
 19. The apparatus as claimed inclaims 1 or 2 or 12 or 13, characterised in that said conduit isprovided with a gas concentration meter (58) for at least one gas type.20. The apparatus as claimed in claim 16, characterised in that saidconduit (50) is connected to an accumulator bottle (62) adapted toreceive gas when the gas concentration passes a predetermined level. 21.The apparatus as claimed in claims 1 or 2 or 12 or 13 characterised inthat said drive circuit (30) is provided with an oxygen conduit (21) forfilling the drive circuit with oxygen, said oxygen conduit beingactivated if required.
 22. The apparatus as claimed in claim 14characterised in that said conduit (50) comprises a buffer (56) toequalize volume variations.
 23. The apparatus as claimed in any one ofclaims 1 or 2 or 12 or 13, one gas type administered to the patientbeing an anaesthetic gas, characterised in that expiration gas whichduring expiration passes the drive circuit side of the exchanger, issupplied through a recovery conduit (70) to an absorber (75) foranaesthetic gas.
 24. The apparatus as claimed in claim 23, characterisedin that the anaesthetic gas is xenon, and that said absorber contains azeolite (76).
 25. The apparatus as claimed in claim 24, characterised inthat the pressure and temperature in said absorber is set to optimisethe absorption of xenon.
 26. The apparatus as claimed in claim 23characterised in that said absorber consists of at least two unitsthrough which the gas can be supplied alternatively.
 27. The apparatusas claimed in claim 9 characterised in that the tubes are filled withcorrugated, folded, and/or laminated material.
 28. The apparatus asclaimed in claim 9 characterised in that said tubes are internallyprovided with a mesh.
 29. The apparatus as claimed in claim 24,characterised in that said absorber consists of at least two unitsthrough which the gas can be supplied alternately.
 30. The apparatus asclaimed in claim 25, characterised in that said absorber consists of atleast two units through which the gas can be supplied alternately.